If `setuptools_scm` is missing, hg-git gets version 0.0.0 silently.

Steps to reproduce

1. Start in some environment that doesn’t have setuptools_scm installed.
2. Unpack 1.0.0 source tarball from PyPI.
3. Run python setup.py build.

Expected result

grep '^Version:' hg_git.egg-info/PKG-INFO should print “Version: 1.0.0”.

Actual result

grep '^Version:' hg_git.egg-info/PKG-INFO prints “Version: 0.0.0”.

Analysis

In setup.py, ImportErrors from import setuptools_scm get silenced. If setuptools_scm is not imported, no version is set.

I think that such bugs should not happen silently. Unless we find a way to make it find the correct version without setuptools_scm, the ImportError should to shown to the user / packager.